Compressor safety device



w 1 3 "Z 31 M March 27, 1962 A. J. GORAND 3,026

COMPRESSOR SAFETY DEVICE Filed Aug. 10, 1959 INVENTOR. ALFRED J. GORAND A TORNEY High Pressure Source United States Patent JCeoglgany, Philadelphia, Pa., a corporation of New Filed Aug. 10, 1959, Ser. No. 832,837 6 Claims. (Cl. 137-13) Tlus invention relates to a method for preventing explosive conditions n compressors used for compressing a free-oxygen containing stream, for example, air compressors.

h Most a1r compressors are lubricated by some type of ydrocarbon 011. Reciprocating compressors require protection of bearings, piston, cylinder walls, piston rod, and valves. Rotary, centrifugal and axial-flow compressors are also lubricated. Reciprocating compressors leak small amounts of oil into the discharge section due to the loose fit between the piston and cylinder walls. Other types of compressors develop leaks in the seals due to wear and high temperatures and pressures.

As a result of these leaks small amounts of very fine 011 droplets atomized by mechanical action and turbulent flow pass through the outlet valves and into the discharge section of the compressor along with the compressed air. These oil particles develop and hold charges of static electricity.

It is believed that a positive charge builds up on the wall surface of the discharge line and that negatively charged oil particles flow along with the air stream. The positive charges flow along the metal surfaces to ground and the negatively charged particles hold their charge. The concentration of negatively charged oil particles may increase to the point that a spark passes through the electric field. This concentration may be very low, on the order of a few parts per million and still result in a dangerous total charge.

Another source of electrical discharge is the compressor itself. The internal metal surfaces of compressors are conductors and it would be expected that condensed electrical charges would flow from surface to surface eventually reaching the ground. However, the internal configuration of some compressors is such that extreme friction is caused by the compressed gas as it flows in turbulent, expanding configuration from the compression section through the exhaust valve and into the discharge section of the compressor. The amount of static electricity condensing on certain surfaces is so great that a suflicient charge builds up and that charge discharges to a conductor as a spark.

Mixtures of hydrocarbon vapors and air reach inflammable concentrations when the concentration of hydrocarbon is in the region between about .5% to 10% by volume. This inflammable concentration occurs in the discharge section of air compressors.

An explosion may occur when a spark is emitted into a hydrocarbon-air mixture of inflammable concentration.

It has now been found that introduction of an atomized liquid antistatic agent into the discharge section of the compressor prevents the building up of an electric field strong enough to emit a spark. The antistatic agent increases the electrical conductivity of the oil mist to such a degree that an electric field strong enough to cause a spark does not develop. In speaking of the discharge section I wish to include the area on the discharge side of the exhaust valve. When the antistatic agent is continually contacting the exhaust valve surface, it will remove condensed static charges from the surface.

Reference is made to the accompanying drawing for the preferred mode of carrying out this invention.

Reference numeral 1 denotes the discharge section of a conventional reciprocating compressor 2. A reciprocating compressor has been shown for illustrative purposes, but the compressor may be any conventional type such as rotary, centrifugal or axial flow. Air from any desired high pressure source is fed through line 3 which contains atomizing means 4. A venturi has been disclosed as the atomizing means in the drawing, however, means which are equivalent for the purpose such as an orifice plate or a globe valve may be used. The antistatic liquid is maintained in closed tank 5 under pressure from lines 6 which connects the high pressure line 3 with the top of the tank. The pressure differential at the atomizing section is therefore the head of antistatic liquid less the pressure loss through the needle valve 7. A pressure differential of .5 to 5.0 p.s.i.g. is adequate depending on flow velocity through the atomizing means which in turn is dependent on the pressure differential between the discharge zone and the high prsesure source. The needle valve is adjusted to feed drops of liquid to the atomizing section so that the concentration of antistatic agent in the discharge section will be from 20 to parts per million (by weight). Reference numeral 8 denotes a quill type feed pipe end. A sparger may be used instead to obtain good distribution of the atomized agent.

Alternative methods may be used to introduce the antistatic agent to the system. For instance, it may be mixed with the oil which lubricates the compressor. As a result any oil which reaches the discharge section will contain the antistatic agent. Another variation is to add the antistatic agent at the intake section of the compressor. The agent will pass through the compressor and then into the discharge section. In the latter embodiment, air velocity and turbulence is relied upon to atornize the antistatic agent and to bring it in contact with the oil droplets.

Many antistatic agents have been developed in the past and have been used in such areas as the preparation of fiber and films having reduced tendency to accumulate static charges. To be suitable for the present process the agent must be stable up to about 200 F. and compatible to some extent with both oil and water. I have found an effective agent to be a mixture comprising 1 to 5 parts of an ester derived from a polyalkylene oxide and an organic acid and 1 part of a quaternary ammonium compound containing 12 to 30 carbon atoms. The polyalkylene oxide may be derived from ethylene or propylene, preferably ethylene oxide. The number of oxide units may vary from 4 to 10, depending upon the other compounds used. Suitable acids are C to C monocarboxylic, alkylaryl sulfonic acids and dibasic acid monoesters (the ester group containing 2 to 8 carbon atoms). A preferred ester is 4-polyoxyethylene stearic acid. Desirable quaternary ammonium compounds are those having the type formula wherein R is an anion, R is an alkyl group of 11 to 20 carbon atoms, R is a lower alkyl radical of 2 to 3 carbon atoms, and R and R are also low molecular weight carbon chains or may be a part of an oxygen containing ring compound. Examples are dodecyl triethyl ammonium sulfate, triamylbutyl ammonium octyl benzene sulfonate and hexadecyl (ethyl) morpholenium etho-sul-fate, the latter being the preferred species.

Ethanolamine and polyethanolamine esters of fatty acids may be used instead of the quaternary ammonium compound, for example triethanolamine stearate.

The antistatic agent is added in amounts in the range of .05 to .25 by weight to assure a concentration of 20 to 150 parts per million in the discharge section.

When the antistatic agent is added directly to the oil which lubricates the compressor surfaces it will be used in conjunction with an antioxidant such as ditertiarybutyl-p-cresol. In addition conventional emulsifying soaps may be used. These include sodium, potassium, calcium, magnesium, barium, cadmium, chromium, copper or zinc. A suitable soap composition contains sodium sulfonates-SO parts, copper sulfonates--25 parts and chromium sulfonates-25 parts.

In the preferred embodiment wherein the antistatic agent is atomized into the air stream passing through the discharge section of the compressor, the agent may be added as an aqueous emulsion. The emulsifier contained in the emulsion promotes contact of the oil particles with the antistatic agent. Emulsifiers that may be used are sodium and potassium mahogany sulfonates (molecular weight 350 to 450), sodium salicylate, disodium alphasulfonaphthenate and n-alkyl gamma hydroxybutylamide. The emulsifying agent is used in the emulsion in concentrations of 0.1% to 1.0% by volume.

Thus the present invention provides a simple and effective method for preventing explosive conditions from occurring in the discharge section of an air compressor. It is important to note that the present method does not do away with the requirement of grounding the compressor.

Modifications may occur to those skilled in the art. For instance, due to the increase in velocity of the air passing through the outlet valve, it may be advantageous to introduce the antistatic agent near the outlet valve to obtain additional dispersion and to remove electrical charges from the valve surface.

Filters may be placed downstream in the discharge line to trap out oil and antistatic agent. This would be necessary when the compressed air is to be used as instrument air.

While the foregoing description has been limited to the prevention of explosions due to the accumulation of static electricity in air compressors, the invention is applicable to units compressing other free oxygen containing gases such as pure oxygen and mixtures of oxygen with other gas or air With other gases.

'1 claim:

1. A method of preventing an explosion in a compressor stream wherein oil which escapes from the working parts of a compressor due to the operation thereof and mixes with a free oxygen containing stream being delivered by said compressor to form atomized oil droplets in the compressor stream, which consists in preventing the accumulation of static electricity on said droplets by introducing an atomized liquid antistatic agent into the discharge section of the compressor.

2. The method according to claim 1 in which the antistatic agent is introduced under pressure.

3. The method according to claim 1 in which the antistatic agent is a mixture comprising 1 to 5 parts of an ester of a polyalkylene oxide to 1 part of a quaternary ammonium compound.

4. The method according to claim 3 in which the ester is 4-poly,oxyethylene stearic acid and the ammonium compound is hexadecyl (ethyl) morpholenium etho-sulfate.

5. The method according to claim 1 in which the antistatic agent is a mixture comprising 1 to 5 parts of an ester of a polyalkylene oxide to 1 part of an ester of a polyethanolamine.

6. The method according to claim 5 in which the first ester is 4-polyoxyethylene stearic acid and the second ester is triethanolamine stearate.

References Cited in the file of this patent UNITED STATES PATENTS Ogint Aug. 12, 1958 OTHER REFERENCES 

